The sintered silicon nitride has hithereto attracted attention in industrial circles as one of the new engineering ceramics for use as a high temperature structural member. Silicon nitride is, however, a compound in which the atoms tend to be bonded together by covalent bonding, and thus it is difficult to sinter as it is. For this reason, compounds of low melting point are generally added to silicon nitride to improve its sintering property.
In most of the prior art processes, sintering agents of oxides have been used such as the oxides of aluminum (Al), magnesium (Mg), yttrium (Y), rare earth elements of lanthanide series such as lanthanum (La) and cerium (Ce), beryllium (Be) and ziroconium (Zr). Further, nitrides and oxinitrides of the above elements have also been used as a sintering agent.
With an ordinary process for the preparation of sintered silicon nitride, however, each compound of the sintering agent is separately added to and mixed with the powder of silicon nitride. It is also known to use as a sintering agent the powder which is abraded from the ball mill.
Even if the mixing of the sintering agent with the powder of silicon nitride is conducted for a long time, however, a uniform mixing or dispersion of the sintering agent cannot be attained by means of anyone of the prior art methods described above, so that a defect would be formed in the resulting product and the scattering results observed with respect to the mechanical properties thereof.
Particularly, it is often necessary to use more than two kinds of sintering agents in order to enhance the sintering property. Accordingly, it becomes all the more difficult to attain a uniform dispersion of the sintering agents in the powder of silicon nitride.